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Cell Calibration
When cell calibration is required, the reference gas passes through a moisture generator where
it sorbs a known quantity of moisture before flowing to the cell. The indicated ∆ƒ (or result-
ing moisture concentration value) can then be compared to the known moisture added to the
reference gas by the moisture generator. Cell calibration is then validated or can be adjusted
by altering the cell span (the system value Moist_Span) as required. Note that the amount of
moisture the sample can sorb from the generator is dependent on flow rate as well as temper-
ature so the sample handling system has been designed to ensure that these two parameters
remain constant.
Refer to the Configuration Data Sheet supplied with the analyzer for a complete listing of fac-
tory set system values including the precise value of the moisture generator in the analyzer.
Refer to (Chapter 4-4): Cell Calibration and Zero Offset for more information on the Zero and
Calibration cycles.
Correction Factor (Coefficient D) Required for Some Applications
Cell Zero Offset (<1ppmv Applications)
Figure 2 illustrates the flow paths of the sample and reference gases in the Model 2850 when
dried sample gas is used as the reference. The two gases travel different paths to the cell and
are, therefore, exposed to different outgassing mechanisms which alter the moisture content
of the gases. This results in an inherent moisture offset between the two analyzer channels
which takes the form of a small cell offset frequency. For extremely low-level applications
(<1 ppmv), this offset must be accounted for - that is, the cell must be zeroed. The cell
offset frequency must be subtracted from ∆ƒ before it is used in the moisture concentration
calculations. This is accomplished by altering the value of coefficient D (the system value
PolyCoeff_D).
Interference Caused by Dissimilar Gases
If the sample and reference gases are not of the same type, chemical and physical differences
between the gases will cause background interference. Like the zero offset, this interference
must be subtracted from ∆ƒ before it is used in the moisture concentration calculations. This is
also accomplished by altering the value of coefficient D.
Additional external valves and sample line connections are necessary to determine the value
of coefficient D. Please refer to Appendix C: Determination of Coefficient D for the required
sample system layout and procedure used to measure the offset and/or interference and alter
NOTE
the value of coefficient D.
1-4 2850 Moisture Analyzer
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